This invention relates to a mechanical plating process and a composition useful in such a mechanical plating process. More particularly, the invention relates to an improved mechanical plating process and composition of the type in which a tenaciously adherent metallic coating is applied on a surface of an object by subjecting metal particles to mechanical energy in a liquid medium to flatten and cold weld the metal particles to the object's surface and plate a continuous adherent metallic coating on the surface. Such mechanical plating processes are described in my earlier U.S. Pat. Nos. RE 23,861; 2,689,808; 2,640,002; 3,023,127; 3,132,043; 3,479,209; 4,202,915; and 4,293,584, which are hereby incorporated by reference.
The inventions disclosed in the foregoing launched a whole new field of plating which was new and unique in that it used mechanical energy to build up a coating instead of thermal or electrical energy. Thus, the mechanical plating process is distinct from electroplating techniques, hot dip plating techniques (e.g., galvanizing) and the like and also from paints, such as zinc rich paints, in which metal particles are adhered to a substrate by means of a paint binder. Over the years, mechanical plating has grown, and commercial plants, numbering in the hundreds, are operating in various countries around the world. In current commercial operations, objects to be coated, plating metal, promoter chemicals and impacting media (such as glass beads) are charged into multi-sided barrels which are rotated horizontally to produce coated objects by tumbling mechanical action within the barrel. The coating, typically 0.00025 inches thick, is satisfactory in all respects. However, using optimum equipment and formulations, a processing time of at least about 30 minutes is ordinarily required. Moreover, barrel plating is limited to the coating of objects small enough to fit within the rotating barrel. In general, therefore, barrel plating is limited to providing relatively thin coatings on relatively small objects and requires relatively long processing times. A more complete description of the mechanical barrel plating process is disclosed in my co-pending application Ser. No. 048,931 filed on May 12, 1987 which is hereby incorporated by reference.
For 30 years or more, mechanical plating has been done in rotating barrels and limited essentially to the same promoter system. The promoter system is the medium employed along with the metal powder to cause plating of the metal powder onto a surface as a result of the application of mechanical energy. The promoter system currently in use in mechanical barrel plating is typically an aqueous solution consisting of a sulfuric acid flux, a mixture of hydrocarbon filming agents or surfactants, and a defoaming agent. The pH of the promoter system is usually adjusted to between 1.2 and 2.0 at the beginning of each batch. In the barrel process the entire solution of promoter is discarded at the end of each batch so that all dissolved zinc salts and contaminants are discarded and as such, each batch begins with an entirely fresh solution of promoter.
It has become extremely desirable to adapt the process of mechanical plating to production of coatings outside of the barrel. However, the adaptation of the mechanical plating process to processes outside of the barrel created new problems which had not been of importance in the barrel plating process. For example, it became a difficult problem to transport the promoter solution and metal powders to the mechanical plating location. So far as is known, no attempt was made to make a promoter specially adapted for plating outside the barrel. Furthermore, the share of the market for barrel plating was further reduced by the inability to plate heavy coatings on very large pieces. Another problem with mechanical barrel plating was that the glass beads used as impact media had a tendency to break down into small pieces when impacted by heavy workpieces tumbling in the barrel. Each glass bead broken by impact created a large number of smaller glass fragments which increased the problem of transporting the zinc to the surface to be coated since the glass fragments made the plating mixture highly impermeable.
Two of my earlier patents, namely U.S. Pat. No. 4,202,915 and No. 4,293,584 related to mechanical plating processes done outside of the barrel. These processes merely adapted the promoter system used in mechanical barrel plating to the specific applications outside of the barrels. However, the use of the mechanical barrel plating promoter outside of the barrel created several new problems. For example, where the plating tool is a circular rotating brush with metal bristles, the tool exerts a very high degree of pressure on each individual wire bristle which, in combination with the hydrocarbon promoter system of the barrel plating process produces a very high polishing action, which creates an ultra-smooth, extremely shiny and lubricated surface. Employing the promoter of mechanical barrel plating such a surface cannot be further plated. This resulted in a limitation on the thickness of the coating to approximately 3.5 to 5 mils. This highly polished coating is known as a Bielby layer and has always been thought to be incapable of accepting a further adherent coating.
Another problem with the use of the mechanical barrel plating chemistry outside of the barrel is that the promoter system requires a pH between 1.0 and 3.0. Such a low pH of the promoter will corrode most equipment which comes in contact with the promoter. Thus, it is desirable to provide a promoter which does not have such an acidic pH and thereby minimizes corrosion problems associated with the equipment used for plating.
Alkaline systems were tried using using caustic soda, washing soda, bicarbonate of soda and an abrasive powder. An excellent quality coating was obtained. With this system it was possible to use a steel wire brush for the first time. There was no change in pH and the mix could be stored for weeks without deterioration. In commercial trials it was received enthusiastically.
It has long been thought that it is impossible to deposit a metallurgically integrated metallic coating on top of heat treat scale, mill scale, any metal surface which has not been cleaned down to the bare metal, or non-metallic surfaces. It does not matter whether the metallic coating is to be applied by hot-dip galvanizing, electroplating or metal cladding, for proper bonding to the substrate, it is thought that all heat treat scale must be completely removed. Usual methods include acid pickling, sand blasting, shot blasting and grit blasting. In all of these processes substantial quantities of the underlying substrate are removed, especially when it is necessary to clean out scale from holes or pits. This forms pollutant salts such as ferrous sulphate and also constitutes an important loss of the metal substrate.
Because of the high cost and difficulties associated with the scale removal on items such as structural steel the scale is usually left in place and covered with a zinc rich paint or frequently a red lead paint. The paint provides a barrier layer to fill in cracks in the scale to thereby provide corrosion protection. It is not comparable to solid metal protective coatings because the paints can be easily scratched or damaged. Substantial advantages would be gained by applying a very heavy zinc coating over scale which is tightly bonded to steel. This has always been considered impossible since it is thought that the scale must be removed prior to applying a coating.